How to Weld Cast Metal

Use special techniques to weld cast iron.

Since aluminum is ductile, it can expand and contract without cracking. Consequently, welding an aluminum casting with the right filler metal is fairly straightforward. Because cast iron cannot stretch or shrink, it is difficult to weld without further cracking. In addition, its high carbon content hardens the metal during the cooling process, similar to heat treating a file. The lower shear strength of the brittle heat-affected zone weakens the cast iron and accelerates the cracking process. If you have a cracked casting, use an arc welder together with special materials and techniques to avoid this problem.

Things You'll Need

• 1/8-inch drill bit
• Cordless drill
• Large propane torch
• Certified temperature indicating crayon
• Fiberglass heat-proof welding blankets
• Arc welder
• 1/8-inch diameter pure nickel welding rods
• Ball-peen hammer

Instructions

1. Drill a 1/8-inch hole right through the casting on both ends of the crack with a cordless drill. This will isolate the fracture and prevent it from spreading further during the welding process.

2. Prepare the crack by grinding out a V-shaped groove along the damaged section with and angle grinder. Ensure that the groove is at least two-thirds the depth of the parent metal and that it extends at least 1/2-inch beyond both ends of the crack.

3. Preheat the casting slowly and evenly with a large propane torch. Scribe lines on the casting with a light-colored certified temperature indicating crayon rated at 1,000 degrees Fahrenheit. When the marks turn black, the casting has reached the correct welding temperate. Note: If the casting cannot be preheated because it is attached to another piece of machinery, use the "back-step and peen" technique described in Step 5.

4. Drape one or two fiberglass heat-proof welding blankets over the entire casting on either side of the repair. This will prevent the metal from cooling too rapidly during the welding process.

5. Set your arc welder to the lowest possible amperage consistent with welding a normal bead with a 1/8-inch diameter pure nickel welding rod. Run a short test bead on a small piece of preheated test metal for proper adjustment; this can be an old casting or a piece of scrap mild steel.

6. Weld 1-inch long anchor beads all the way along the base of the groove, using the "back-step" method and a pure nickel welding rod. Start about an inch from the end of the crack and run the bead toward the start of the groove. Move another inch away from the start of the first bead, and back-step the weld to meet the first run.

7. Peen each bead flat with a ball-peen hammer immediately after every run. This will stretch the filler metal as it cools, thereby minimizing stresses caused by the shrinkage.

8. Apply the same "back-step and peen" technique and continue adding filler metal on either side and on top of the anchor bead until the groove is full.

9. Cover the weld area with another welding blanket immediately after finishing in order to allow the casting to cool down as slowly as possible.

Tips & Warnings

• Remove all traces of slag from each inch-long bead with a chipping hammer and wire brush before proceeding with the following run.

• If you have a large casting such as an engine block, keep testing the metal with the temperature indicating crayon. When the casting begins to cool down during welding, remove the blanket and apply the propane torch to bring it back to pre-heat temperature.

• If you're repairing a smallish casting, such as an exhaust manifold, dispense with the welder's blanket, but bury the casting in clean dry sand immediately after welding to retard cooling.

• If you are using a DC welding generator, set the machine to "reverse polarity" by attaching the welding handle to the negative pole and attaching the earth clamp to the positive pole. This will further minimize heat input during welding.

References

• Lincoln Electric: Guidelines for Welding Cast Iron
• Elefan Weld: How to Weld Cast Iron
• Caltech Engineering Services: Temperature Indicating Sticks

Resources

• Esab: Welding Handbook